Surface elevation profiles

1. Method

Surface elevation profiles across the whole width of the glacier were measured in two places. The upstream profile is located in a region where flow stripes are particularly prominent and the downstream profile is as far toward the Ross Ice Shelf as the available photography allows. Both profiles are shown on a Landsat MSS image on Figure 1. On this Landsat image, flow stripes appear to originate in an area 50 km to 120 km upstream of the upstream profile, where no aerial photography is available. Points in the profiles were chosen to represent what was judged to be the “general ice surface”, that is, measurements on prominent crevasses or on seracs or narrow ridges were avoided. The upstream elevation profile is 22.7 km long and consists of about 200 points with an average spacing of 110 m. The downstream profile is 35.4 km long with about 220 points 160 m apart on the average.

Fig. 1.

Landsat-MSS image of Byrd Glacier, from figure 25 of Reference SwithinbankSwithinbank (1988). The locations of the upstream (UU’) and downstream (DD’) photogrammetric profiles are shown.

The measurements were made on an analytical stereo plotter (Wild BC 1) on selected stereoscopic models from photographs taken 56 days apart, 6 December 1978 and 31 January 1979. The photographs have been used for a detailed study of surface motion of Byrd Glacier (Reference BrecherBrecher, 1986) and were taken with a standard wide angle mapping camera (Wild RC 8) at an average scale of approximately 1 : 49 000. Control points for setting up the stereo models came from the aerial triangulations of the two blocks of photographs for the previous surface motion study. Ten to 15 such points appear in each model and, with a few exceptions, all were used in each orientation, resulting in much larger than usual redundancy. Excellent results were obtained in absolute orientations, typically 7 μm in photo y-parallaxes. RMS residuals on the ground are 0.6 m to 0.8 m in plan and 1.1m in elevation.

Four models, one each from adjacent flight lines, were used to span the glacier for each elevation profile. There was considerable side overlap between models in each case and parts of the profiles are therefore duplicated in adjacent stereo models. A line was drawn on one of the diapositives for each model to mark the location of the profiles, and points reasonably evenly spaced along this line were then measured independently in each model. That is, no attempt was made to measure identical points in the overlapping regions, but nevertheless the overlapping elevation profiles are virtually identical. In two cases, although elevation differences also agreed very well, there were small systematic elevation offsets between models which could not be satisfactorily accounted for. Four of the six overlaps are between 3 km and 6 km long and contain at least 30 points each. The very good agreement of the independently measured profiles in these places suggests that the surface has been represented accurately. Using a conservative estimate of 10μm for precision of measurements on the photographs yields precision of 0.5 m for positions and 1.1m for elevations in the terrain. Precision of elevation differences along the profiles is thus 1.1 √2 =1.6m.

2. Results

Uncontrolled mosaics of the upstream and downstream profiles are shown in Figs. 2a and 3a respectively. Corresponding elevation profiles are shown in Figs. 2b and 3b. Flow stripes appear on the photographs as bands of different crevasse texture and brightness pattern. From Figure 2a the general crevasse orientation of the upstream profile corresponds to that of splaying crevasses formed by lateral shear and longitudinal extension. However, the detailed structure of crevasses within flow stripes is largely chaotic, which is probably a result of the complex stress history. Stereoscopic inspection shows that bright stripes in the photographs appear snow-covered and are generally associated with valleys, while dark stripes appear as bare ice areas that usually correspond to ridges.

Fig. 2.a.

Uncontrolled mosaic of aerial photographs showing the upstream profile of Byrd Glacier. The rectangle in the middle of the profile corresponds to Figure 4, where velocity measurements were carried out. b. Surface elevation of the upstream profile.

Fig. 3.a.

Uncontrolled mosaic of the downstream profile. b. Surface elevation of the downstream profile.

The upstream region has a large crevasse density and in general consists of bare ice. Net ablation occurs in this area probably associated with strong katabatic winds (Reference BromwichBromwich, 1992). Flow stripes here appear as bands each with a distinct crevasse pattern. Each stripe ranges from 0.1km to 2 km in width. In the upstream profile there is a long-wavelength component (5 km) of surface elevation with ridge-valley double amplitudes up to 45 m. Superimposed on the long-wavelength trend is a short-wavelength variation with wavelengths of 250 m to 1 km, double amplitudes of about 10 m and slopes of 2–7%. Narrow stripes are steeper than wide stripes. It is these short-wavelength variations that correspond in many cases with flow stripes. There are, however, several flow stripes in the upstream profile that do not correlate with any surface elevation change. This observation contrasts with the traditional view that flow stripes correspond to “gently undulating ridges and troughs” (Reference Crabtree and DoakeCrabtree and Doake, 1980).

In the downstream profile flow stripes appear as bands of smooth ice adjacent to bands of crevassed ice each of which are 1 km to 2 km wide. Crevasses are much less prominent than in the upstream profile and are largely filled with snow. This filling may be because lateral shear is much smaller (Reference BrecherBrecher, 1986) and crevasses are not opening as quickly, or because net accumulation occurs. There is a better correlation between flow stripes and elevation than at the upstream profile, and the slopes associated with stripes range from 1–7%. The short wavelength component of the elevation profile is reduced compared to the upstream profile. The predominating wavelength (λ) is 1.5–4km, with a double amplitude of 20–40m.

Velocity profile

The origin and persistence of flow stripes is unclear. An hypothesis is that flow stripes are formed where two ice tributaries converge and would therefore be associated with shear margins (Reference Casassa, Jezek, Turner and WhillansCasassa and others, 1991). Large lateral shear occurs in shear margins, which could account for the crevasses observed in flow stripes. Crevassing would result in bands of less dense ice, which would be topographically higher once isostasy at depth is achieved. However, positive relief elements tend to collapse. Perhaps flow stripes are continually created, which means that lateral shear might occur everywhere that such stripes are observed.

In order to test this suggestion, a velocity transect was measured across some especially prominent flow stripes on the upstream profile. Forty measurements of displacements were carried out in the area shown with a rectangle in Figure 2a. This area is shown enlarged in Figure 4. Measurements are spaced an average of 47 m apart over a total distance of 1.8 km. A stereo pair from each photo epoch is used. The photographs were taken 56 days apart. The selected site for closely-spaced determinations of velocity is at the center of the glacier. This is a region where lateral shearing is expected to be small. Yet flow stripes with strong topographic relief are prominent.

Errors of displacements are combinations of the errors across-flow coordinate. The velocity measurements are spaced about 47 m, and so the standard error in strain rate is typically 0.001 d⁻¹, but can be as large as 0.006 d⁻¹ for shear computed over closely-spaced measurements.

Fig. 4.

Aerial photograph showing the points used for measuring velocities (black dots) in the upstream profile. The rectangle corresponds to that shown in Figure 2a. The solid line drawn across the photograph indicates the location of the upstream profile. The large circles are used for indicating the position of control points.

Fig. 5.

Longitudinal component of velocity and lateral shear along the profile of Figure 4. The origin of the distance is the same as the one used for Figure 2b, that is, the left margin of the glacier. Vertical arrows show the position of flow stripe boundaries from Figures 2a and 4.

The speed and lateral shear are shown in Figure 5. The average speed of the points in the profile is 1.94md⁻¹. Fluctuations up to 0.1 md⁻¹ about the mean speed are observed across the profile, but no significant in each position measurement and the error in the positions of the two stereomodels with respect to each other. The second error is estimated to be 3 m (Reference BrecherBrecher, 1986), but it is systematic and does not affect velocity differences. The probable uncertainty of relative velocities was computed as 0.01 md⁻¹ based on the measurement precision (0.6 to 0.8 m times √2) and the time interval (56 d ± 3h). Lateral shear ∂v/∂y is computed based on the longitudinal component of velocity (v), y being the variation occurs at the boundaries of flow stripes. Lateral shear is not significantly different from zero across the profile, and there is no special variation at flow stripe boundaries.

Fig. 6.

Near infrared AVHRR image of 2 December 1987, 1659 UTC, showing flow stripes on Byrd Glacier and the Ross Ice Shelf to the right. The profile across Byrd Glacier corresponds approximately to profile DD’ on Figure 1. The image is 127 km by 194 km.

The flow stripes originate 80 km upstream from the upstream velocity profile (Fig. 1). The present results show that lateral shear is not continuing to create the flow stripes. Some other mechanism such as squeezing of bands of different mechanical properties by lateral compression or a very slow decay rate must account for the stripes.